Measurement of Charmless Hadronic B Decays Branching Fraction at Babar

We present preliminary measurements of the branching fractions for charmless hadronic decays of B mesons into two-body final states with kaons, pions and a φ resonance. The measurements are based on a data sample of approximately 23 million BB̄ pairs collected by the BaBar detector at the PEP-II asymmetric B Factory at SLAC. Contributed to the Proceedings of the 36th Rencontres De Moriond On QCD And Hadronic Interactions 17-24 Mar 2001, Les Arcs, France Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 Work supported in part by Department of Energy contract DE-AC03-76SF00515. Charmless hadronic final states play an important role in the study of CP -violation. In the Standard Model, all CP -violating phenomena are a consequence of a single complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix . Measurements of the rates and CP asymmetries for B decays into the charmless final states ππ and Kπ can be used to constrain the angles α 2 of the unitarity triangle. Decays containing a φ meson are interesting since they are dominated by b → s(d)s̄s peguin diagram, with potential benefits to estimates of direct CP violation effects. They also allow an independent measurement of sin 2β. We present preliminary measurements of the branching fractions for charmless hadronic decays of B mesons in the final states ππ, Kπ, Kπ, Kπ, Kπ, φK, φK, and φK(892) . The data sample used in these analyses consists of 22.57 × 10 BB pairs, collected at the PEP-II ee collider (SLAC) with the BaBar detector 4 . Hadronic events are selected based on track multiplicity and event topology. We use only good quality tracks: tracks are identified as pions or kaons using the Cherenkov angle θc measured by a unique, internally reflecting Cherenkov ring imaging detector (DIRC). Candidate K S mesons are reconstructed from pairs of oppositely charged tracks that form a well-measured vertex and have an invariant mass within 3.5 standard deviations (σ) of the nominal K S mass . Candidate photons are defined as showers in the EMC that have the expected lateral shape and are not matched to a track. Candidate π mesons are formed from pairs of photons with an invariant mass within 3σ of the nominal π mass. The π candidates are then kinematically fitted with their mass constrained to the nominal π mass. All tracks (except K S daughters) are required to have good-quality θc measurements that are inconsistent with the expected value for a proton. Electrons are rejected based on specific ionization (dE/dx) in the DCH system, shower shape in the EMC, and the ratio of shower energy to track momentum. Candidate B mesons are reconstructed in the various topologies: hh, hπ, K S h, K S π, φh, φK S , and φK where the symbols h and h refer to π or K. For φ candidates, both tracks must be identified as kaons whose invariant mass must lie within a ±30MeV/c interval centered around φ mass. The selection of K comprises Kπ and K S π combinations within a Kπ mass interval of ±150MeV/c. A K helicity angle cut effectively requires π momentum greater than 0.35GeV/c. Candidate B mesons are selected exploiting the kinematic constraints provided by the Υ (4S) initial state: we define a energy-substituted mass mES, where √ s/2 is substituted for candidate’s energy, and the difference ∆E between the B-candidate energy and √ s/2. For all modes the mES resolution is dominated by the beam energy spread and is approximately 2.5MeV/c, while ∆E resolution is mode dependent and dominated by momentum resolution. Candidates are selected in the range 5.2 < mES < 5.3GeV/c . Candidates are accepted, depending on the decay topology, in various ∆E ranges, which are restrictive enough to suppress backgrounds due to other types of B decays. The largest source of background is from random combinations of tracks and neutrals produced in the ee → qq continuum (where q = u, d, s or c). In the CM frame this background typically exhibits a two-jet structure. In contrast, the low momentum and pseudoscalar nature of B mesons in the decay Υ (4S) → BB leads to a more spherically symmetric event. This topology difference is exploited using two event-shape quantities: the angle θS 6 between the sphericity axes, evaluated in the CM frame, of the B candidate and the remaining tracks and photons in the event. The distribution of the absolute value of cos θS is strongly peaked near 1 for continuum events and is approximately uniform for BB events. We require | cos θS| < 0.9 The second quantity used in the analyses is a Fisher discriminant F which consists of a linear combination of several variables that distinguish signal from background . The experimental observables used in the definition of F are the scalar sum of the momenta of all tracks and photons (excluding the B candidate daughters) flowing into nine concentric cones centered on the thrust axis of the B candidate, in the CM frame. Each cone subtends an angle of 10 and is folded to combine the forward and backward intervals. The global detection efficiencies, which include the intermediate particle branching fractions, are listed in Table 1. Table 1: Summary of results for detection efficiencies (ǫ), numbers of fitted signal yields (NS), statistical significances, and measured branching fractions (B). ( 90% confidence level upper limits). The efficiencies include the branching fractions for K → K S → π π, π → γγ, φ → KK. Equal branching fractions for Υ (4S) → BB and BB are assumed. Mode ǫ (%) NS Stat. Sig. (σ) B(10) ππ 45 41 ± 10 4.7 4.1± 1.0 ± 0.7 Kπ 45 169± 17 15.8 16.7 ± 1.6± 1.3 KK 43 8.2 −6.4 1.3 < 2.5 ππ 32 37 ± 14 3.4 < 9.6 Kπ 31 75 ± 14 8.0 10.8 −1.9 ± 1.0 Kπ 14 59 −10 9.8 18.2 +3.3 −3.0 ± 1.7 KK 14 0.0 −0 0 < 2.5 Kπ 9.6 17.9 −5.8 4.5 8.2 +3.1 −2.7 ± 1.1 φK 18 31.4 −5.9 10.5 7.7 +1.6 −1.4 ± 0.8 φπ 19 0.9 −0.9 0.6 < 1.4 φK 6 10.8 −3.3 6.4 8.1 +3.1 −2.5 ± 0.8 φK 5 4.5 9.7 −3.4 ± 1.7 φK K+ 2.5 7.1 −3.4 2.7 12.8 +7.7 −6.1 ± 3.2 φK K0 2.4 4.4 −2.0 3.6 8.0 +5.0 −3.7 ± 1.3 Signal yields are determined from an unbinned maximum likelihood fit using mES, ∆E, F , θc, φ mass, and K mass (where applicable). In each of the fits, the likelihood for a given candidate j is obtained by summing the product of event yield nk and probability Pk over all possible signal and background hypotheses k. The nk are determined by maximizing the extended likelihood function L: